The present invention is directed towards a bus accessing system in which a plurality of user terminals communicate with each other via a plurality of bus interface units (BIUs) which are connected to a common transmission bus.
In known bus communication systems, each of a plurality of BIUs is associated with at least one user terminal and periodically places information from its associated user terminal on the transmission bus for transmission to the remaining BIUs. The BIU formats the user information into one or more information packets, which generally include: a source address (the address of the BIU associated with the user terminal); a destination address (the address of the user terminal for which the information is intended and its associated BIU); the information to be transmitted; and various other information, such as parity or CRC bits, etc. Once the BIU has properly formatted the information, the BIU places the information packet on the bus for transmission to the remaining BIUs, at times determined by the particular bus accessing method being used. One such bus accessing method is known as a time-division multiple access (TDMA) system, wherein each user terminal is assigned a specific time slot during which it can gain access to the transmission bus. Exemplary of such systems is U.S. Pat. No. 3,851,104. Another major access method is a contention system which uses a listen-while-talk protocol. In this system, the BIU continually monitors the condition of the transmission bus and places information packets on the bus whenever the bus is silent. If, during a transmission, the BIU detects other information on the bus, it halts its transmission for a variable time period and then attempts to regain access to the bus when the line is silent. Examplary of such systems are U.S. Pat. Nos. 4,210,780 and 4,063,220.
Once the formatted packet is placed on bus, it is examined by each of the remaining BIUs to determine if the information contained in the packet is for one or more of the user terminals associated with that BIU. If the packet is for one of its associated user terminals, the BIU reformats the received packet into a form which can be accepted by its user terminal and applies the information to the user terminal at time intervals consistent with the needs of the specific user terminal.